Cheap uk package with a data bundle means they can use the phone, no need for wifi and plenty of free options out there to use to ensure no further costs, if they are both iphones they can facetime etc, the possibilities are endless. No idea what the options are but a pay as you go phone, or sim only plan if the Irish smart phone is unlocked will reduce the costs even further. There is also the possibility of keeping the Irish phone, finding a wifi hotspot and again using the free apps available.
I travel a fair bit with work, mainly to the US and use hotel wifi and skype - no roaming charges or call charges, but thats ok for a few days, if this is a long term thing then its not ideal, you may not be able to reach each other if there was an emergency